Lean Condition Causes? - GP1200R

Looking a reason why my middle cylinder seized. Oil lines still attached, took carbs off and can see oil was flowing from pump to all three cylinders, looked at jets and they are fine. Going to replace Pump Diaphragm (since its apart), but it has no visible tears.

So, it appears that it was getting fuel and had oil. See pic of piston. Why would this happen? Looks like it got super hot on the piston top. Anything else I need to check before I put it back together? I don't want this to just happen again.

An air leak or spark plug not tight enough would be my guess based on what you've described. I've seen a few that changed out spark plugs and didn't torque them enough. It ran awesome right up until it siezed.